You've got the infamous Chevy steering knock. Don't worry we all have it, and it's not going to hurt anything on your truck. I actually hear they may have just come out with a fix for it Just do a search for "steering shaft'' and the like and you'll get all the info you need on this.

I stole this from another site for your reading pleasure, if you can feel it in the steering wheel this is your problem, a new Intermediate shaft is now out, I believe it can be had for around $120. I'll be having mine replaced soon and my truck only has 12k miles on an '03...... hope this helps, d.

Attention: This Service Bulletin DOES NOT include Mid-Size Utilities such as Buick Rainier, Chevrolet TrailBlazer Models, GMC Envoy Models or Oldsmobile Bravada. Refer to Service Bulletin 02-02-35-006A or newer for Mid-Size Utilities.

Condition
Some customers may comment on a clunk-type noise coming from under the hood that also can be felt in the steering wheel. These conditions may be more noticeable when turning at low speeds on rough road surfaces.

Diagnostic Tips
Use the information below to help diagnose the source of the noise.

• Frame Snap and/or Popping Type Noise -- A frame snap or popping type noise can be duplicated on rough or smooth road surfaces with steering wheel input to the left or the right. This type of noise can be HEARD and is typically louder with the windows rolled down. For additional information, refer to Corporate Bulletin Number 03-08-61-002F or newer -- Snap/Popping Type Noise Coming from Front of Vehicle (Remove Front Crossmember, Change Fastener Orientation).

Replace the I-Shaft with P/N 19153614.
Do NOT lubricate or exercise the I-Shaft.

Important:

• I-shaft P/N 19155614 has been designed to replace previous designed dampened and non-dampened I-shafts. The physical difference in the yoke size will accommodate all vehicles listed in this bulletin.

• Due to the design of the new I-shaft, it is not possible to lubricate/grease the I-shaft.

Replace the steering column upper intermediate shaft with an improved design shaft that will eliminate the clunk noise using the procedure listed below.

Set the front wheels in the straight ahead position.
Notice: On the 2002 and later model year vehicles, the steering column LOCK was removed from the steering column. It is critical that the J 42640 - Steering column Anti-Rotation Pin is used when servicing steering columns on 2002 and later model year vehicles. Failure to use the J 42640 may result in damage to the SIR coil.

Set the steering wheel in the LOCK position on 2001 and prior model year vehicles.

For 2002 and later vehicles, install the J 42640 in the steering column lower access hole.

From under the hood, remove the lower bolt that connects the upper intermediate shaft to the steering gear coupling shaft.
Slide the shaft towards the dash in order to disengage the shaft from the steering gear coupling shaft.
For vehicles equipped with adjustable foot pedals, perform the following steps:
6.1. Reposition the carpet away from the accelerator pedal position (APP) sensor.

6.2. Remove the two nuts retaining the accelerator pedal to the bulkhead.

6.3. Reposition the accelerator pedal out of the way so the intermediate shaft can be removed.

From inside the vehicle, remove the upper bolt from the upper intermediate steering shaft (1) to the steering column connection.
Remove the upper intermediate steering shaft assembly.
8.1. From inside the vehicle, slide the shaft down and off the steering column.

8.2. From inside the vehicle, slide the upper intermediate shaft through the dash boot seal and remove the shaft from the vehicle.

Replace the upper intermediate shaft.
Install the upper intermediate steering shaft through the dash boot seal and slide the lower end into the steering gear coupling shaft.
Raise the upper end of the intermediate steering shaft and install into the steering column shaft.
Install the upper bolt and nut.
Tighten
Tighten the bolt to 47 N·m (35 lb ft).

What you're describing sounds like it could also be hydraulic cavitation, or air in the PS lines. Look around the base of the steering pump for signs of moisture, meaning a leak of PS fluid. Also the obvious, check the PS fluid level.

I had the same thing and all it took was removing the shaft bolt and grinding it shorter. Its a particular prob once u change headres or have some work done for some reason. I don't think its any major fix...check the shaft bolt first, as its prob just knocking and/or catching on the headre tubes.